Split starlette and fastapi imports and error msg

Since all three apps A2AStarletteApplication, A2AFastAPIApplication, and
JSONRPCApplication are public APIs, we should assume that each can be
used direcly by downstream projects. Therefore, proper ImportError
messages should be printed when initializing each type of app with
missing optional dependencies.

Author: darkhaniop

src/a2a/server/apps/jsonrpc/fastapi_app.py

@@ -5,14 +5,20 @@
 
 if TYPE_CHECKING:
     from fastapi import FastAPI, Request, Response
+
+    _package_fastapi_installed = True
 else:
     try:
         from fastapi import FastAPI, Request, Response
+
+        _package_fastapi_installed = True
     except ImportError:
         FastAPI = Any
         Request = Any
         Response = Any
 
+        _package_fastapi_installed = False
+
 from a2a.server.apps.jsonrpc.jsonrpc_app import (
     CallContextBuilder,
     JSONRPCApplication,
@@ -51,6 +57,12 @@ def __init__(
               ServerCallContext passed to the http_handler. If None, no
               ServerCallContext is passed.
         """
+        if not _package_fastapi_installed:
+            raise ImportError(
+                'The `fastapi` package is required to use the `A2AFastAPIApplication`.'
+                ' It can be added as a part of `a2a-sdk` optional dependencies,'
+                ' `a2a-sdk[http-server]`.'
+            )
         super().__init__(
             agent_card=agent_card,
             http_handler=http_handler,

src/a2a/server/apps/jsonrpc/jsonrpc_app.py

@@ -47,22 +47,21 @@
     from starlette.requests import Request
     from starlette.responses import JSONResponse, Response
 
-    _http_server_installed = True
+    _package_starlette_installed = True
 else:
+    FastAPI = Any
     try:
-        from fastapi import FastAPI
         from sse_starlette.sse import EventSourceResponse
         from starlette.applications import Starlette
         from starlette.authentication import BaseUser
         from starlette.requests import Request
         from starlette.responses import JSONResponse, Response
 
-        _http_server_installed = True
+        _package_starlette_installed = True
     except ImportError:
-        _http_server_installed = False
+        _package_starlette_installed = False
         # Provide placeholder types for runtime type hinting when dependencies are not installed.
         # These will not be used if the code path that needs them is guarded by _http_server_installed.
-        FastAPI = Any
         EventSourceResponse = Any
         Starlette = Any
         BaseUser = Any
@@ -145,9 +144,11 @@ def __init__(
               ServerCallContext passed to the http_handler. If None, no
               ServerCallContext is passed.
         """
-        if not _http_server_installed:
+        if not _package_starlette_installed:
             raise ImportError(
-                'The `a2a-sdk[http-server]` package is required to use the `JSONRPCApplication`.'
+                'Packages `starlette` and `sse-starlette` are required to use the'
+                ' `JSONRPCApplication`. They can be added as a part of `a2a-sdk`'
+                ' optional dependencies, `a2a-sdk[http-server]`.'
             )
         self.agent_card = agent_card
         self.extended_agent_card = extended_agent_card

src/a2a/server/apps/jsonrpc/starlette_app.py

@@ -6,14 +6,21 @@
 if TYPE_CHECKING:
     from starlette.applications import Starlette
     from starlette.routing import Route
+
+    _package_starlette_installed = True
+
 else:
     try:
         from starlette.applications import Starlette
         from starlette.routing import Route
+
+        _package_starlette_installed = True
     except ImportError:
         Starlette = Any
         Route = Any
 
+        _package_starlette_installed = False
+
 from a2a.server.apps.jsonrpc.jsonrpc_app import (
     CallContextBuilder,
     JSONRPCApplication,
@@ -52,6 +59,12 @@ def __init__(
               ServerCallContext passed to the http_handler. If None, no
               ServerCallContext is passed.
         """
+        if not _package_starlette_installed:
+            raise ImportError(
+                'Packages `starlette` and `sse-starlette` are required to use the'
+                ' `A2AStarletteApplication`. It can be added as a part of `a2a-sdk`'
+                ' optional dependencies, `a2a-sdk[http-server]`.'
+            )
         super().__init__(
             agent_card=agent_card,
             http_handler=http_handler,